1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile device, etc.
2. Description of the Background Art
There exist various types of image forming apparatuses which utilize the Carlson process, such as a copying machine, a printer, a facsimile device, etc. In such image forming apparatuses, an image carrier is formed of, for instance, a photosensitive drum, which is rotated by a motor and is charged uniformly by a charging unit. Thereafter, image exposure is performed on the image carrier by use of an exposing unit and thereby an electrostatic latent image is formed. The latent image is developed by a developing unit with two-components developer or one-component developer in order to obtain a toner image, and the toner image is transferred to a transfer paper fed by a paper feeding apparatus by use of a transferring unit. The transferred image is then fixed by a fixing apparatus. And further, in a two-sided copying mode, after forming an image on a front surface of the transfer paper (after transferring and fixing the toner image as mentioned above), the same processes of forming an image on the rear surface of the transfer paper are executed.
As to transfer mediums, there typically exist two types of transfer mediums, which are, a contact-type transfer medium employing a transfer conveying belt, a transfer roller, etc., and a non-contact transfer medium employing a charger, etc. The transfer conveying belt rotates with a same linear velocity as that of the image carrier. A transfer bias is applied to the transfer conveying belt from a high voltage power source for transferring, and the belt thus biased electrostatically sucks and conveys the transfer paper transported from the paper feeding apparatus, and further the toner image formed on the image carrier is transferred to the transfer paper.
In a case that the development medium employs two-components developer including toner and carrier, the mixing ratio (toner density) of the toner and the carrier of the two-components developer is detected by a toner density sensor. In a toner density controlling portion, the necessity of supplementing the toner for the two-components developer is judged from comparing the detection value of the toner density sensor with a toner density controlling standard value, and in accordance with the result thereof the supplementing of toner is controlled.
In such a construction, the supplementing of the toner from a toner supplementing portion to the two-components developer in the developing means is controlled. In such an image forming apparatus employing the Carlson process, when the image is formed, the electrostatic force is utilized on the respective processes, and thereby the image quality and the transfer paper transporting quality are apt to be affected by variations environmental conditions, such as temperature and humidity.
For instance, in the developing method employing the two-components developer including toner and carrier as the developing medium, although both of the toner and carrier are charged by electrostatic force caused by mutual friction, the electrostatic force varies due to the environmental conditions, such as temperature and humidity. As is apparent from experience, the electrostatic force is apt to occur in the low-temperature environment, while the same is not apt to occur in the high-temperature environment. In order to keep the image density (the density of the toner image) constant on the image carrier, since the electrostatic force caused by the friction between toner and carrier varies due to the environmental conditions, e.g., temperature-humidity, it is necessary to change the toner density of the two-components developer in the developing medium in accordance with changes in the environmental conditions, e.g., temperature-humidity.
Regarding solutions for solving the subject matter as mentioned heretofore, there have been proposed a method of reading out image density on the image carrier and changing the toner density controlling standard value on the basis of the read-out value, and another method of changing the toner density controlling standard value in accordance with the temperature and humidity as measured by a temperature/humidity sensor in the image forming apparatus carrying the temperature/humidity sensor.
And further, a water-containing rate of the transfer paper varies in accordance with the humidity. In a high-temperature/high-humidity environment, when the transfer paper absorbs aqueous vapor, the paper feeding efficiency and the transferring property are considerably lowered, and thereby a non-transportation (a phenomenon of not feeding the transfer paper from the paper feeding portion) occurs or a state of a poor transferring occurs on the image surface. As one proposed solution, such a defect has been solved by utilizing an aqueous vapor removing heater for drying out the paper.
Furthermore, there exists an image forming apparatus in which a temperature/humidity sensor is installed and various controls are performed by the output signal of the temperature/humidity sensor, or another image forming apparatus requiring control by the output signal of the temperature-humidity sensor. In a low-cost image forming apparatus, however, a system for effectuating various controls based on the outputs signal of the temperature/humidity sensor is omitted for the sake of cost reduction.
Japanese Laid-open Patent Publication No. 2-12385/1990 describes an image forming apparatus for roughly calculating a resistance value of a transfer roller and controlling a transferring condition (charging) based on the detected resistance value. Japanese Laid-open Patent Publication No. 4-63385/1992 describes an image forming apparatus having a humidity sensor provided therein and controlling a transferring condition on the basis of an output signal of the humidity sensor.
And further, in recent years, based on the technology trend of conserving the global environment, many devices have implemented a contact transfer medium employing a transfer conveying belt, a transfer roller, etc., which generate very little ozone, instead of the conventional non-contact type transfer medium employing a charger. Regarding the transfer conveying belt, Japanese Laid-open Patent Publication No. 62-203169/1987 and Japanese laid-open Patent Publication No. 63-83771/1988 describe such a belt. All of these above-mentioned apparatuses are formed with a construction dispersing carbon black, metal powder, pure cotton cloth, etc., in the conveying belt as an electrically conductive substance, in order to obtain an intermediate substantial resistance value by use of an elastic rubber material for the transfer conveying belt.
Furthermore, in the transfer conveying belt employing the elastic rubber material, there is a method of utilizing a conductivity of a polymer, as another method of giving the rubber semiconductivity.